A semiconductor LASER is widely used for read-out and write-in from/to an optical disc. Storable amount of information per unit area of an optical disc is inversely proportional to the second power of a wavelength of the semiconductor LASER. Thus, it is effective to shorten the LASER wavelength in order to increase a storage density. Gallium nitride has a wide forbidden band gap of 3.4 eV, is a direct gap semiconductor, and can be mixed with nitride aluminum and nitride indium to produce a mixed crystal. Therefore, it is easy to produce a double-hetero structure of a semiconductor junction. Gallium nitride is thus expected as a material for a short wavelength LASER such as 400 nm or so.
Further, gallium nitride has a wide forbidden band gap and a high dielectric breakdown voltage such as 5.times.106V/cm, as well as a high electron saturation drift speed such as 1.5.times.107 cm/s, and those features encourage gallium nitride to be a material for a high-speed transistor operative at a high temperature.
In a crystal growth of gallium nitride system material, hetero-epitaxial growth using a sapphire as a substrate is employed in general, because no excellent substrate made of gallium nitride is available. In this crystal growth, a metal organic vapor phase epitaxy (MOVPE) method or a molecular beam epitaxy (MBE) method is employed in general. In addition, the halide VPE method has been introduced recently and draws attention as a tool for improving a crystallinity, where ammonia is used as a nitrogen material, and hydrogen chloride gas passed over the heated gallium surface is used as a gallium material for growing a thick film. This halide VPE method can grow a film at a growth rate of 100 micron/hour. R & D employing this method has been actively engaged on film growth technology.
In a conventional method of forming gallium nitride crystal using the halide VPE method, sapphire is used as a substrate material and a gallium nitride crystal having a film thickness of not less than 100 micron is formed. As FIGS. 10(a) and 10(b) depict, a sapphire substrate 5 is heated to 1000.degree. C., and ammonia gas is reacted with gallium chloride which is produced by chloride gas passed over the surface of metal gallium (not shown) heated to 850.degree. C., thereby forming gallium nitride 9 having over a 100 micron thickness on the sapphire substrate 5.
In this conventional method, however; the density of a growth nuclear formed on the sapphire substrate 5 is low, and the growth rate is high due to the high temperature of the substrate, as a result, three-dimensional growth becomes in majority. The surface of the gallium nitride crystal thus formed is rough, and a crystal dislocation density is high due to a large number of crystal grains, accordingly, it is difficult to obtain a gallium nitride crystal having excellent flatness as well as crystallinity through this conventional method.